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mapreduce uses @simd which can cause different results depending on optimizations #59657
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as discussed on triage, this is a result of LLVM reassocation that is explicitly made legal by
Base.add_sum. As such,math-mode=ieeecauses this to return the same result. I think the only thing to potentially do here is to document the functions in Base which may return different results on different architecutres, and people that always want consistent results between systems need to usemath-mode=ieeeThis is also different results based on different inference results, not just different systems. e.g. you could get one result with a production implementation and then add an
@showstatement which hurts inlining and inference and consequently get a different result. Then the presence of@show(or other spooky action at a distance that impacts compiler heuristics) would change the returned value.The element type thing is different, andEdit: oh both are type instabilities — I think this is something we should definitely fix; and #58418 addresses it. The fact that we can commute + but not other operators is a lot trickier. We could get consistent results if nothing ever commuted, with a ~1.5-2x perf regression.Are we sure that reassociation is the root cause? What about f being a non-const global, could that prevent inlining (which would lead to reassocation being blocked)?
- Reacted by Sukera and Oscar Smith
yes.
So is there anything to do here?
I don't think so. (other than docs)
I don't think so. (other than docs)
I'm not convinced. I don't think we should be using
@simdhere. I think we can get close enough to peak performance without it to make this worthwhile.- changed the title
[-]Refferential transparency broken in floating point sum[/-][+]`mapreduce` uses `@simd` which can cause different results depending on optimizations[/+]on Sep 29, 2025 Updated the title at least.
Reacted by Sukera
This produced incorrect answers in a real-world workflow. This is a simplified MWE:
$ julia --startup=no _ _ _ _(_)_ | Documentation: https://docs.julialang.org (_) | (_) (_) | _ _ _| |_ __ _ | Type "?" for help, "]?" for Pkg help. | | | | | | |/ _` | | | | |_| | | | (_| | | Version 1.11.5 (2025-04-14) _/ |\__'_|_|_|\__'_| | Official https://julialang.org/ release |__/ | julia> x = Float32[429.17752, 429.2375, 429.29248, 430.21002, 430.995, 431.16498, 431.025, 430.995, 430.995, 430.995, 430.77252, 430.575, 430.255, 430.0, 430.375, 430.535]; julia> f = identity identity (generic function with 1 method) julia> sum(x -> identity(x), x) 6886.6f0 julia> sum(x -> f(x), x) 6886.6006f0 julia> versioninfo() Julia Version 1.11.5 Commit 760b2e5b739 (2025-04-14 06:53 UTC) Build Info: Official https://julialang.org/ release Platform Info: OS: Linux (aarch64-linux-gnu) CPU: 64 × Neoverse-N1 WORD_SIZE: 64 LLVM: libLLVM-16.0.6 (ORCJIT, neoverse-n1) Threads: 64 default, 0 interactive, 32 GC (on 64 virtual cores) Environment: JULIA_CONDAPKG_BACKEND = Null JULIA_CONDAPKG_EXE = /home/ubuntu/temple/.venv/bin/python JULIA_PROJECT = @. JULIA_NUM_THREADS = auto